Late variscan tectonic evolution by thinning of earlier thickened crust. An Ar---Ar study of the Montagne Noire, southern Massif Central, France     

Henri Maluski  Sylvie Costa  Helmut Echtler 《Lithos》1991,26(3-4):287-304

⁴⁰Ar---³⁹Ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the Montagne Noire (the southern part of the Massif Central, France).

This range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high T/medium P conditions. The basement is structurally overlain by epimetamorphic Paleozoic sediments in the north. In the south, equivalent rocks form a well-known pile of recumbent nappes. The Variscan history of this area comprises early shortening and thickening of the continental crust. The climax of this event took place around 320 Ma ago, as is shown by previous Rb---Sr data and by ⁴⁰Ar---³⁹Ar measurements. New developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core. South-southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts. This movement was accompanied and followed by dextral extensional wrenching of the Paleozoic cover along the northern and southern margins of the axial zone. Attendantly, ductile shear zones were formed.

In order to decipher the complex history, three structural levels have been dated with the ⁴⁰Ar---³⁹Ar method. These levels are: (1) The migmatitic and gneissic series in the axial core, where micas yield ages of c. 315 Ma; (2) Mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c. 310 Ma; (3) Muscovites and biotites from the epimetamorphic nappe structures yielding ages of c. 297 Ma. The later ages may represent a younger thermal pulse. These data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints. A detailed geodynamic evolution of the studied segment is proposed.  相似文献   

Averaging two functions     

Helmut Moritz 《Journal of Geodesy》1987,61(1):21-40

The paper treats the problem of averaging two independent determinations of the same function (curve, surface, etc.). Starting from the simple arithmetic mean, the problem is successively generalized to scalar and vector functions, arriving at an integral equation of Wiener-Hopf type. Aspects of least-squares collocation are also included. Finally the problem of determining transformation parameters and other systematic deviations is treated, furnishing a simple example of continuous least-squares adjustment.  相似文献   

The holstein interglaciation: Time-stratigraphic position and correlation to stable-isotope stratigraphy of deep-sea sediments     

Michael Sarnthein  Helmut E. Stremme  Augusto Mangini 《Quaternary Research》1986,26(3)

Marine molluscan shells from para-type and other loclities of the Holsteinian interglaciation were dated by Th/U and the electron spin resonance (ESR) method to more than 350,000 and 370,000 yr B.P., beyond the limit of Th/U dating. The high age estimate is corroborated by a K/Ar age of 420,000 yr B.P. determined from volcanic ash near the base of the Ariendorf paleosol in the Middle Rhine valley believed to be a pedostratigraphic equivalent of the Holsteinian. Shells from the Herzeele marine unit III, an equivalent of the Wacken (Dömnitz) warm stage in northern France and subsequent to the Holsteinian, revealed ages between 300,000 and 350,000 yr B.P. A correlation of these two warm stages with marine oxygen-isotope stages 11 and 9 on the SPECMAP and CARTUNE time scales is suggested. From the benthic oxygen-isotope record one may infer that no exceptionally high global sea-level rise corresponds to the large transgressions of the Holstein Sea in northern Germany. Therefore, a significant proportion of the transgression was probably the result of an unusually large local glacial-isostatic depression caused by the extreme buildup of ice during the preceding Elster glaciation (stage 12). According to the deep-sea record, it lasted approximately 50% longer than the subsequent cold stage 10. The outstanding soil formation with Braunlehm and the well-developed thermal optimum of the Holsteinian are tentatively related to a phase of minimum sea-ice cover in the Norwegian-Greenland Sea, as deduced from long benthic carbon-isotope records from the central Atlantic.  相似文献   

Analysis of cyclical sedimentation in the lower Permian Mchuchuma coalfield (South-West-Tanzania)     

Prof. Dr. Satyendra M. Casshyap  Dr. Thomas Kreuser  Prof. Dr. Helmut Wopfner 《International Journal of Earth Sciences》1987,76(3):869-883

A statistical approach by a modified Markov process model was used to prove that the lower Permian coal measures of the Tanzanian Mchuchuma basin developed distinct cyclicities during deposition. From results the transition path of lithologic states typical for this coal sequence is as follows: (A) coarse sandstone, (B) medium sandstone, (C) fine-very fine sandstone, (D) shale, (E) mudstone, (F) coal or (E+F) and again (A) coarse sandstone. The majority of cycles is are asymmetric (ABCDEF-ABC.)but symmetrical cycles are present as well (ABCDEFED). The statistical results concur with observed sedimentological evidence of depositional environment. Fining upward cycles correspond to the development of basal channel bars, changing into levées, then into back swamps which are topped by coal swamps. Frequent interbedding of coal and mudstone in the upper part of the coal measures calls for periodic flooding and flushing of fine clastics into coal-forming swamps causing interruption of peat formation. Cyclical deposition is explained by wandering channels in response to varying discharge and rate of deposition, their lateral frequent shifting caused the formation of asymmetrical cycles in this fluviodeltaic depositional model.

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Zusammenfassung Das kohleführende untere Perm des Mchuchuma Beckens (SW-Tanzania) wurde statistisch mit Hilfe der Markov Analyse auf Zyklizität innerhalb der Abfolge untersucht. Die statistischen Daten zeigen, daß die bevorzugte und für dieses Kohlebecken typische Sequenz folgende lithologische Abfolge umfaßt: (A) grober Sandstein, (B) mittlerer Sandstein, (C) feiner Sandstein, (D) Siltstein, (E) Tonstein, (F) bzw. (E+F) Kohle und erneut (A) grober Sandstein. Es existieren meist asymmetrische Zyklen (ABCDEF-ABCD...) aber symmetrische Zyklen (ABCDEFED) treten ebenfalls auf. Die Markov Analyse ergänzt sehr gut die Beobachtungen bezüglich des Ablagerungsmilieus in den Aufschlüssen. »Fining up« Zyklen beginnen mit basalen Gerinnesanden, überlagert von levée Ablagerungen, welche ihrerseits übergehen in »back swamps« und Kohlesümpfe. Der obere Teil der Kohleabfolge ist durch häufige Wechsellagerungen von Kohle mit Tonstein charakterisiert, die auf periodische Überflutungen und Verfrachtungen von Feinklastika in die Kohlesümpfe zurückzuführen sind. Wandernde Flußgerinne, ausgelöst durch rasch wechselnde Sedimentfracht und Ablagerungsrate, führten zu zyklischen Ablagerungen. Häufig auftretende laterale Gerinneverlagerungen erklären die asymmetrischen Zyklen in diesem fluvio-deltaischen Ablagerungsmodell.

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Résumé Des analyses statistiques ont été effectuées par la méthode de Markov sur le Permien inférieur charbonnier du bassin de Mchuchuma (Tanzanie du sud-ouest) dans le but de montrer l'existence de cycles sédimentaires. Les résultats font apparaître une séquence, typique de ce bassin, qui s'ordonne ainsi de bas en haut: (A) grès grossier, (B) grès moyen, (C) grès fin à très fin, (F) siltite, (E) pélite, (F) charbon ou (E+F) et de nouveau (A) grès grossier. Les cycles sont pour la plupart assymétriques (ABCDEF-ABC...) mais il en existe aussi de symétriques (ABCDEFED).Les résultats obtenus par la méthode de Markov concordent avec les observations sédimentologiques relatives au milieu de dépôt. Les cycles granoclassés débutent par des sables fluviatiles surmontés par des dépôts de levées suivis à leur tour de »back swamps«, et se terminent par des dépôts de marais tourbeux. La partie supérieure de la série est caractérisée par une stratification alternée de charbon et de pélite due à des inondations périodiques amenant dans les marais des produits clastiques fins qui interrompaient la formation de la tourbe. La sédimentation cyclique s'explique par la migration de chenaux, provoquée par les variations de l'apport sédimentaire et de la vitesse de dépôt. Les fréquents déplacements latéraux des chenaux expliquent l'asymétrie des cycles, dans ce modèle fluvio-deltaïque.

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- Mchuchuma (-). , : (A) ; (B) ; (C) - ; (D) ; (E) ; (F) , (E + F) (A) . (ABCDEF-ABCD), (ABCDEFED). . « » , , , . (). , , , . / .

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Post-drilling destabilization of temperature profile in borehole Yaxcopoil-1, Mexico     

Jan Šafanda  Philipp Heidinger  Helmut Wilhelm  Vladimír Čermák 《Hydrogeology Journal》2007,15(2):423-428

As part of the International Continental Scientific Drilling Program (ICDP), the 1.5-km-deep borehole Yaxcopoil-1, located in the Chixculub meteor impact structure in Mexico, has undergone further study after drilling operations ceased. Temperature logs were repeated ten times at intervals 0.3–0.8, 15, 24 and 34 months after borehole shut-in. The logs bear a distinct signature of transient heat transfer by groundwater flow manifested by a gradual distortion of the linear temperature profile when a cold wave of 0.8–1.6°C amplitude was detected propagating downward from 145 to 312 m at a rate of 4–6 m/month. To understand the nature of this moving anomaly, a 20-day monitoring of the cold wave was carried out at a depth of 307 m that showed further cooling of 0.6°C during the first 16 days of the passage followed by temperature stabilisation. As an explanation of this unusual phenomenon, a theory is proposed, whereby the drilling mud has accumulated within the overlying and cooler highly porous and permeable karstic rocks during the drilling and migrates downward. The observed migration rate suggests a permeability higher than 10^?11 m². This indicates a high vulnerability to contamination of the only freshwater aquifer in the Yucatan region.  相似文献   

Revision of the Conchostracan Genus Tenuestheria from the Upper Cretaceous Lanxi Formation in Zhejiang and Its Biostratigraphic Significance in Southeast China   总被引：1，自引：0，他引：1       下载免费PDF全文

LI Gang  WAN Xiaoqiao  Helmut WILLEMS  David J. BATTEN 《《地质学报》英文版》2007,81(6):925-930

The diagnosis of the conchostracan genus Tenuestheria from the Lanxi Formation in Jinhua, Zhejiang Province in Southeast China is revised following an examination of the type species under a scanning electron microscope, which revealed some morphological features on the carapace that had not been recognized previously. The importance of the Tenuestheria Fauna is considered in the context of correlation of six Turonian–Santonian formations in this region of China. The value of the Cenomanian Nemestheria and Turonian Linhaiella faunas in correlating the underlying early Late Cretaceous formations is also noted.  相似文献   

Leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions   总被引：14，自引：0，他引：14  

Thorsten Geisler  Marcus Ulonska  Helmut Schleicher  Robert T. Pidgeon  Wilhelm van Bronswijk 《Contributions to Mineralogy and Petrology》2001,141(1):53-65

We report results of hydrothermal experiments on four alluvial zircons from Sri Lanka, which cover a wide range of radiation damage, at 450 °C and 1.3 kbar for 744 h with 2 M CaCl₂ solution as reactive fluid. After the hydrothermal treatment, the most metamict samples show micrometer-thick reaction rims, which surround apparently unreacted zircon, as revealed by cathodoluminescence (CL) and Nomarski differential interference contrast (NDIC) images. These rims have sharp, curved, and transgressive boundaries with unreacted zircon and are, in some cases, spread out along cracks. The thickness of reaction rims increases with increasing cumulated !-dosage of the starting materials. The reaction rims are strongly enriched in Ca (up to 7000 ppm) and a water species and depleted in radiogenic Pb, Zr, and Si, as revealed by electron microprobe analyses. A significant Th loss from the reaction rims was detected in the case of the most metamict sample, whereas U remained in the structure. FT-infrared spectrometry and X-ray diffraction measurements revealed that the bulk run products were recrystallized. Using micro-Raman spectrometry, we were able to demonstrate that differential recrystallization took place. The reaction rims are strongly recrystallized, whereas the unreacted grain interiors underwent only minor recrystallization. Recrystallization of the rims is accompanied by an enhancement of the integral CL intensity. It is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions. Previous zircon studies have shown that (1) a transgressive morphology, (2) a reduced Th-U ratio, and (3) an enhanced CL emission are also characteristics of rims in zircons from high-grade metamorphic rocks. Based on these similarities between natural and experimentally produced rims, it is suggested that leaching-catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core-rim structures found in natural zircons. Furthermore, the strong enrichment of Ca in the reaction rims supports previous assumptions that high Ca concentrations in natural zircons are of secondary origin. It is suggested that lower U-Pb concordia intercept ages obtained from single-phase zircons with high Ca contents date a leaching event.  相似文献   

Stockholm, Rio and beyond: Lessons from two decades of international environmental politics     

Helmut Breitmeier 《Climatic change》1996,32(1):1-6

Conclusion Studies that include moderate climate forecasts, farmer adaptation, carbon fertilization, and warm-loving crops tend to show that climate change will have only mild impacts on average global agricultural output and may even improve temperate agricultural production. On this point, recent studies yield strikingly consistent results. Of course, impact estimates still contain uncertainties. Key questions include how agriculture might change by 2060, how tropical and subtropical farming will be affected, and how effects will be distributed regionally. The most likely threat to agriculture from climate warming is regional damages in relatively poor areas that lack either the knowledge or the financial resources to adjust. Although it is not clear which regions will actually suffer, the ones that are most vulnerable lie predominantly in or near the tropics (IPCC, 1995). Nonetheless, on average, the factors discussed in this essay will help mitigate the impact of climate change on agriculture.The authors are grateful to the Electric Power Research Institute (EPRI) for financial support. We also wish to thank Richard Adams, Cynthia Rosenzweig, Kathleen Segerson, Joel Smith, Robert Unsworth and Thomas Wilson for their helpful comments. The authors alone are responsible for any remaining errors or omissions.  相似文献   

Gezielte Tiefsee-Beobachtungen: Eine neue Tiefsee-Fernsehkamera mit eingebauter Fotokamera und mit gekoppelten Sammelgeräten     

Günter Dietrich  Helmut Hunger 《Ocean Dynamics》1962,15(6):229-242

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Die Heutigen numerischen grundlagen zur reduktion von triangulationsmessungen auf das ellipsoid     

Helmut Wolf 《Journal of Geodesy》1963,37(2):149-163

Ohne Zusammenfassung Communication présentée à l'Assemblée générale de l'Association Internationale de Géodésie (Helsinki 1960).  相似文献